Correctly distinguish between multiple values, and multiple allowances

[kbknapp]

This is a slightly breaking change - not enough to warrant a label.

Currently, using named values -f <file> <mode> requires you to set multiple(true)...but this isn't correct logic. By this token, someone could legally do:

$ myprog -f some.txt fast -f other.txt slow

This may, or may not, be what the developer wanted. Setting multiple(true) shouldn't be a requirement for named values...only for unnamed multiple values. This allows the developer to decide which style of argument he wants, multiple allowances, or multiple values.